In recent years, the use of both positron emission tomography (PET) and magnetic resonance imaging (MRI) for medical diagnosis has become more widespread. While MRI is an imaging method for displaying structures and slice images in the interior of the body, PET allows visualizing and quantifying of metabolic activities in vivo.
PET uses the particular properties of positron emitters and positron annihilation to quantitatively determine the function of organs or regions of cells. Appropriate radiopharmaceuticals marked with radionuclides are administered to the patient before the examination. When the radionuclides decay, they emit positrons which within a short distance interact with an electron and this results in a so-called annihilation. This creates two gamma quanta which fly apart in opposite directions (offset by 180°). The gamma quanta are detected by two PET detector modules lying opposite one another within a particular time frame (coincidence measurement), as a result of which the location of the annihilation is determined to be at a position along the connecting line between these two detector modules.
For the purpose of detection, the detector module in PET must in general cover a large portion of the gantry arc length. It is subdivided into detector elements with a side length of a few millimeters. Each detector element generates an event record specifying the time and detection location, that is to say the corresponding detector element, if it detects a gamma quantum. This information is transmitted to a fast logic and compared. If two events coincide within a maximum period of time, a gamma decay process is assumed to have occurred on the connecting line between the two corresponding detector elements. A tomography algorithm, that is to say the so-called back projection, is used to reconstruct the PET image.
U.S. Pat. No. 7,218,112 B2 discloses a combined PET/MRI system which uses lutetium oxyorthosilicate (LSO) as a scintillation crystal for converting the gamma quanta into light and uses avalanche photodiodes (APD) for detecting the light. The photodiodes are connected to preamplifiers.
The gain of commonly used semiconductor amplifiers and semiconductor detectors (avalanche photodiodes, APD) in particular depends on the temperature. Since the components are subjected to temperature variation, in particular heating, during operation, cooling is necessary. The temperature of the amplifiers and photodiodes can be controlled globally by supplying cooled air. When using air with a constant temperature, the temperature of the amplifiers results from the balance of the generated heat and the heat emitted through the air via the surfaces of the amplifiers. The cooling can be used in the same fashion for other parts of the detection system.
Spanoudaki et al., “Effect of Temperature on the Stability and Performance of an LSO-APD PET Scanner”, IEEE Nuclear Science Symposium Conference Record 2005, 3014-3017 investigated the temperature stability of the LSO APDs of a PET system. The influence of different temperature sources on different components of the PET system is analyzed therein. By way of example, the gain of the APD depends on the temperature, as does the position of the respective photopeak. By way of example, the temperature and the position of the photopeak over time are illustrated in a number of graphical illustrations. The change of the energy resolution with temperature is analyzed and statistics for the 256 examined detection modules are generated. In this case, the causes of the temperature increase are investigated and a prediction for the shift of the photopeak and energy resolution with increasing temperature is made.
US 20040071259 A1 discloses an x-ray detector with a thermoelectric cooling unit. Both active and passive cooling units are used in this case. A reference temperature is stabilized by controlling the power of the thermoelectric cooling unit by determining the temperature of the x-ray detector by way of temperature sensors.
It is disadvantageous in the case of the known solutions that the ability to control the temperature is insufficient if there are changes in the heat generation during operation. By way of example, if there is an increase in the generated heat or in the heat introduced from the outside (for example by turbulence in the RF screen induced by gradients), then the gain will change despite the cooling because the above-mentioned balance has shifted. Furthermore, there are typically different sources which input heat. This input of heat results in a spatial and temporal temperature distribution. This can result in the temperature-dependent components such as APDs and preamplifiers having a different temperature. Due to the fact that it is typically not possible to directly measure the temperature of the critical components, and that a plurality of components influence the gain of the overall chain via their temperature dependence, the individual temperature profile must be inferred from spatially distributed temperature measurement points. As an alternative, it is possible to omit this intermediate step and directly determine the relationship with the gain.